Polypropylene films have been widely used in electrical applications because of, for example, their excellent electrical characteristics. In particular, their use as a dielectric material in capacitor applications is significantly increasing. In recent years, there has been a strong demand for miniaturization and lower prices, and the thickness of a film that acts as a dielectric has become increasingly thinner. In addition, required withstand voltage characteristics and processability into elements have also become severe.
Further, for flat capacitors typified by a safety standard capacitor, there has been an increasing demand for reducing the noise due to film oscillation (acoustic pressure oscillation). This noise occurs upon oscillation of a dielectric such as a film wound around a capacitor element due to an alternating electric field applied. This noise notably occurs particularly upon high-frequency input and pulse input.
To improve the withstand voltage characteristics and processability into elements, the film surface needs to be moderately roughened. This is important particularly to improve slip characteristics of the film. Further, this is particularly important for providing preservability in a metallized capacitor. The preservability is such a function that, in a metallized capacitor in which a metallized film that serves as an electrode is formed on a dielectric film, the deposited metal is dispersed by discharge energy at the time of abnormal discharge to restore the insulation characteristics and prevent a short circuit, thereby maintaining the function of the capacitor or preventing breakdown. The preservability is an extremely useful function also from the standpoint of safety.
In capacitors used under application of an AC voltage, when the surface roughness a film is large, corona discharge occurs between the film layers and corona breakdown between the layers is likely to occur. Thus, it is necessary to narrow a gap between the film layers. If the gap between the film layers is narrowed, film oscillation due to application of an alternating electric field can be suppressed to reduce noise. Thus, it is important to narrow an interval between the film layers also for reducing noise.
Thus, to achieve high withstand voltage characteristics and suitable processability into elements and noise characteristics, it is necessary to narrow an interval between film layers to suppress corona generation and noise while roughening the surface of the film to obtain preservability and slip characteristics.
Examples of the method for roughening a film surface proposed include mechanical methods such as embossing and sandblasting, chemical methods such as chemical etching using an solvent, the method of stretching a sheet in which a dissimilar polymer such as polyethylene is mixed, the method of stretching a sheet in which β-crystals are formed, and the like (JP 51-63500 A and JP 2001-324607 A).
However, the mechanical and chemical methods result in low roughness density, and the method of stretching a sheet in which β-crystals are formed is likely to form bulky projections. In addition, films the surface of which has been roughened by these methods have problems in that, for example, a gap between film layers are ununiform, and at sites where the gap have been widened by the bulky projections, corona discharge is likely to occur when an AC voltage is applied, resulting in a reduced capacitor lifetime and degraded noise characteristics as well. The method of stretching a sheet in which a dissimilar polymer such as polyethylene is blended has a problem of poor recyclability, that is, although few bubbles are left when a capacitor is formed, the dissimilar polymer can have an adverse effect when the film is recycled.
As a film having a uniform roughness density on the film surface and a uniform projection size, a high melt strength polypropylene film has also been proposed (JP 2001-72778 A). A film formed by laminating the high melt strength polypropylene film and a normal polypropylene film has also been proposed (JP 2001-129944 A).
However, there is a problem in that when a high melt strength polypropylene resin itself is used in capacitor applications, sufficient heat resistance or pressure resistance cannot be obtained because of the structure of the resin, and the dielectric breakdown voltage particularly at a high temperature significantly decreases. In addition, when using a technique of laminating high melt strength polypropylene resins, it is very difficult to obtain a structure with uniform laminate thickness particularly in the case of a thin film with a film thickness of 5 μm or less, and a practically satisfactory dielectric film cannot be provided because of impaired uniformity.
A biaxially stretched polypropylene film the roughness of the film surface of which is controlled and a method for producing the same have also been disclosed (Japanese Patent No. 3508515).
However, this technique is insufficient for obtaining uniform projection density while narrowing the interval between layers on both film surfaces.
A technique for forming a film surface with a fine roughness and balancing element winding characteristics and pressure resistance by controlling the β-crystal fraction of a cast original sheet within a certain range to thereby has also been disclosed (Japanese Patent No. 3508515 and JP 54-53253 A). In Japanese Patent No. 3508515 and JP 54-53253 A, roughness on one surface of a film is defined.
However, roughness on both surfaces of a film cannot be sufficiently controlled by that production method, and fine surface roughness of the resulting film cannot provide high withstand voltage characteristics and noise characteristics at the same time.
On the other hand, with regard to noise, a proposal for minimizing the volume between each layer of a wound film to enhance adhesion between the films has been made. For example, the method of taking up a capacitor element under reduced pressure has been proposed (JP 54-53253 A). Applying a corona discharge treatment to both surfaces of a polypropylene film has also been proposed (JP 61-145812 A).
However, in the method of winding an element under reduced pressure, the process for winding a capacitor element is complicated, resulting in decreased productivity. Further, when the element is placed back in air after winding the capacitor element, reintrusion of air between the film layers cannot be prevented. In the method of applying a corona discharge treatment to both surfaces of a film, blocking can occur before the process of winding up into a capacitor element, for example, in the vacuum deposition process or the like.
Further, it has been proposed that the thermal shrinkage stress value at 120° C. and heat seal strength in the longitudinal direction is optimized to obtain a film for a capacitor that shows a small decrease in electrical capacitance when a voltage has been applied in a high temperature environment for a long period of time and decreases the noise of the capacitor (JP 2009-088492 A).
However, although the noise characteristics and the occurrence of corona discharge can be inhibited, only the optimization of the thermal shrinkage stress value at 120° C. and heat seal strength in the longitudinal direction cannot ensure sufficient preservability of the capacitor because of too strong adhesion between the films or provide high withstand voltage characteristics.
It could therefore be helpful to provide a biaxially oriented polypropylene film that has high withstand voltage characteristics, suitable processability into elements, and excellent noise characteristics particularly in AC voltage capacitor application, and to provide a metallized film and a film capacitor using the biaxially oriented polypropylene film.
We thus provide:
(1) A biaxially oriented polypropylene film having projections on both surfaces of the film, wherein the height of the most common projection (PhZ) among the projections present on each surface is not less than 100 nm and less than 400 nm on both surfaces, and the number of projections per 0.1 mm2 (Pc) on each surface is not less than 150 and less than 500 on both surfaces.
(2) A metallized film formed by providing at least one surface of the biaxially oriented polypropylene film with a metal film.
(3) A film capacitor using the metallized film.
By having a film surface with a number of low-height projections on both sides, a biaxially oriented polypropylene film having high withstand voltage characteristics, suitable processability into elements, and excellent noise characteristics particularly in AC voltage capacitor application can be provided. Further, a metallized film and a film capacitor using the biaxially oriented polypropylene film can be provided.